CoffeeScript Quick Ref.pdf

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1. CoffeeScript Quick Reference ￿ coffeescript.org ￿ ￿ General1 · Whitespace is significant · Ending a line will terminate expressions - no need to use semicolons · Semicolons can be used to fit multiple expressions onto a single line · Use indentation instead of curly braces { } to surround blocks of code in functions, if statements, switch, and try/catch · Comments starts with # and run to the end of the line Functions · Functions are defined by an optional list of parameters in parentheses, an arrow, and an optional function body. The empty function looks like: -> · Mostly no need to use parentheses to invoke a function if it is passed argu- ments. The implicit call wraps forward to the end of the line or block expres- sion. · Functions may have default values for arguments. Override the default value by passing a non-null argument. Objects and arrays · Objects and arrays are similar to JavaScript · When each property is listed on its own line, the commas are optional · Objects may be created using indenta- tion instead of explicit braces, similar to YAML · Reserved words, like class, can be used as properties of an object without quot- ing them as strings Lexical Scoping and Variable Safety · Variables are declared implicitly when used (no var keyword). · The compiler ensures that variables are declared within lexical scope. An outer variable is not redeclared within an in- ner function when it is in scope · Using an inner variable can not shadow an outer variable, only refer to it. So avoid reusing the name of an external variable in a deeply nested function · CoffeeScript output is wrapped in an anonymous function, making it diffi- cult to accidentally pollute the global namespace · To create top-level variables for other scripts, attach them as properties on window, or to exports in CommonJS. Use: exports ? this Splats · Splats ... can be used instead of the variable number of arguments object and are available for both function def- inition and invocation Loops and Comprehensions · Comprehensions for ... in work over arrays, objects, and ranges · Comprehensions replace for loops, with optional when guard clauses and the value of the current array index: for value, index in array · Array comprehensions are expressions, and can be returned and assigned · Comprehensions may replace each/forEach, map or select/filter · Use a range when the start and end of a loop is known (integer steps) · Use by to step in fixed-size increments · When assigning the value of a compre- hension to a variable, CoffeeScript col- lects the result of each iteration into an array · Return null, undefined or true if a loop is only for side-effects · To iterate over the key and value prop- erties in an object, use of · Use: for own key, value of object to iterate over the keys that are directly defined on an object · The only low-level loop is the while loop. It can be used as an expression, returning an array containing the result of each iteration through the loop · until is equivalent to while not · loop is equivalent to while true · The do keyword inserts a closure wrap- per, forwards any arguments and in- vokes a passed function Try/Catch/Finally · try/catch statements are as in JavaScript (although expressions) If, Else, Unless, and Conditional Assignment · if/else can be written without paren- theses and curly braces · Multi-line conditionals are delimited by indentation · if and unless can be used in postfix form i.e. at the end of the statement · if statements can be used as expres- sions. No need for ?: Chained Comparisons · Use a chained comparison to test if a value is within a range: minimum < value < maximum Array Slicing and Splicing with Ranges · Ranges can be used to extract slices of arrays · With two dots [3..6], the range is in- clusive (3, 4, 5, 6) · With three dots [3...6], the range ex- cludes the end (3, 4, 5) · The same syntax can be used with as- signment to replace a segment of an ar- ray with new values, splicing it · Strings are immutable and can not be spliced Embedded JavaScript · Use backquotes `` to embed JavaScript code within CoffeeScript 1 E. Hoigaard © 2554/2011 Rev. α
2. Everything is an Expression · Functions return their final value · The return value is fetched from each branch of execution · Return early from a function body by using an explicit return · Variable declarations are at the top of the scope, so assignment can be used within expressions, even for variables that have not been seen before · Statements, when used as part of an ex- pression, are converted into expressions with a closure wrapper. This allows as- signment of the result of a comprehen- sion to a variable · The following are not expressions: break, continue, and return Operators and Aliases · CoffeeScript compiles == into ===, and != into !==. There is no equivalent to the JavaScript == operator · The alias is is equivalent to ===, and isnt corresponds to !== · Logical operator aliases: and is &&, or is || and not is an alias for ! · In while, if/else and switch/when statements the then keyword can be used to keep the body on the same line · Alias for boolean true is on and yes (as in YAML) · Alias for boolean false is off and no · For single-line statements, unless can be used as the inverse of if · Use @property or @method instead of this.something · Use in to test for array presence · Use of to test for object-key presence Existential Operator · Use the existential operator ? to check if a variable exists. ? returns true unless a variable is null or undefined · Use ?= for safer conditional assignment than ||= with numbers or strings · The accessor variant of the existential operator ?. can be used to soak up null references in a chain of properties · Use ?. instead of the dot accessor . in cases where the base value may be null or undefined. If all of the properties ex- ist then the expected result is returned, if the chain is broken, then undefined is returned instead Classes, Inheritance, and Super · Object orientation as in most other ob- ject oriented languages · The class structure allows to name the class, set the superclass with extends, assign prototypal properties, and define a constructor, in a single assignable expression · Constructor functions are named as the class name, to support reflection · Lower level operators: The extends operator helps with proper prototype setup. :: gives access to an object’s prototype. super() calls the immediate ancestor’s method of the same name · A class definition is a block of exe- cutable code, which may be used for meta programming. · In the context of a class definition, this is the class object itself (the constructor function), so static properties can be assigned by using @property: value, and functions de- fined in parent classes can be called with: @inheritedMethodName() Destructuring Assignment · To make extracting values from com- plex arrays and objects convenient, CoffeeScript implements destructuring assignment · When assigning an array or object lit- eral to a value, CoffeeScript breaks up and matches both sides against each other, assigning the values on the right to the variables on the left · The simplest case is parallel assignment [a,b] = [b,a] · It can be used with functions that return multiple values · It can be used with any depth of array and object nesting to get deeply nested properties and can be combined with splats Function binding · The fat arrow => can be used to define a function and bind it to the current value of this · This is helpful when using callback- based libraries, for creating iterator functions to pass to each or event- handler functions to use with bind · Functions created with => are able to ac- cess properties of the this where they are defined Switch/When/Else · The switch statement do not need a break after every case · A switch is a returnable, assignable ex- pression · The format is: switch condition, when clauses, else the default case · Multiple values, comma separated, can be given for each when clause. If any of the values match, the clause runs String Interpolation, Heredocs, and Block Comments · Single-quoted strings are literal. Use backslash for escape characters · Double-quoted strings allow for inter- polated values, using #{ ... } · Multiline strings are allowed · A heredoc ''' can be used for formatted or indentation-sensitive text (or to avoid escaping quotes and apostrophes) · The indentation level that begins a here- doc is maintained throughout, so the text can be aligned with the body of the code · Double-quoted heredocs """ allow for interpolation · Block comments ### are similar to heredocs, and are preserved in the gen- erated code Extended Regular Expressions · Extended regular expressions are de- limited by /// and are similar to here- docs and block comments. · They ignore internal whitespace and can contain comments Aliases and : && or : || not : ! is : == isnt : != yes : true no : false on : true off : false Miscellaneous · Twitter comments to @autotelicum http://autotelicum.github.com/Smooth-CoffeeScript/